1. Field of the Invention
This invention relates to a rotary grinding tool, particularly to an endless grinding belt for a belt grinding machine, used preferably for grinding molded wood products.
2. Description of the Prior Art
Wood products, for example, furniture rails and other contoured furniture components, are ground with grinding machines. The grinding machines have rotary grinding tools or else are configured as oscillating grinders. Grinding machines that have rotary grinding tools work either with rotating grinding disks or with rotary endless grinding belts.
The rotating grinding disks that are used for grinding wood profiles consist of a disk body, that have the contour of the profile that is to be ground molded into their circumference. An abrasive, for example, emery cloth, is attached to the circumference surface that is contoured in this manner. The useful life of the abrasive is dependent on the circumferential length of the grinding disk. The greater the grinding circumference, the higher the useful life that can be achieved. However, there are limits to both the disk circumference and thus the useful life, since at the motor speeds that are usually used and the larger diameter grinding disks cutting speeds become too high. Smaller grinding disk diameters such as are necessary for grinding relatively small radii on the wood profiles result, in the other hand, in very short useful lives. Grinding with grinding disks also entails the disadvantage that it is not possible to achieve satisfactory quality in the grinding as a result of the relatively small contact surface of the grinding disk on the wood profile that is to be ground.
Contrary to this, in belt grinding machines that operate with an endless rotary grinding belt it is possible to achieve a longer useful life of the grinding belt. In contrast to that achieved with grinding disks, the quality of the grinding is improved and the wood profiles that are to be ground can pass through the belt grinding machines at relatively higher feed rates. Normally, belt grinding machines are provided with continuous work-piece feed mechanisms for this reason. Belt grinding machines used to grind profiled and straight work-pieces have a replaceable pressure foot that bears the outline of the profile that is to be ground and this is usually arranged between two guide rollers for the grinding belt at approximately equal height. As the profile that is to be ground is fed through the machine this pressure foot presses the rotary grinding belt into the contoured pressure foot. In order to limit the application pressure for the grinding process the contoured pressure foot my also have an automatic springing system. The endless grinding belt consists of a flexible fabric that is coated with abrasive, said grinding belt being relatively stiff for production reasons and also because it must be tightened and guided through the grinding machine.
In order that curved work-pieces can also be ground in a belt grinding machine, in place of the pressure foot a replaceable guide roller is configured as a pressure roller and provided on the circumference with the contour of the profile that is to be ground. The work-piece that is to be ground, as is the case when a pressure foot is used, must press the tightened and thus extremely stiff grinding belt into the contours of the guide and pressure roller, and this is only possible in the case of contoured shapes having larger radii and smaller contour depths.
For this reason, a belt grinding machine cannot grind contoured shapes that are both convex as well as concave in one and the same work step. Because of feed problems and the stiffness of the conventional grinding belts a grinding machine can only grind parts or sectors, respectively, of the profile contour when complicated profile forms are involved. For this reason it is usual to use very long grinding machines that incorporate a plurality of several grinding sets arranged in sequence, these being tiltable up to 135.degree. in order to be able to grind the whole profile contour is one pass. These multi-grinding machines incur very high levels of investment and need very long set-up times.
A further disadvantage can be seen in the fact that the contoured pressure foot soon loses its profile contour as a result of friction on the back of the rotary grinding belt and for this reason has to be replaced or renewed, respectively, very frequently. Pressure feet that do not have a costly mechanical springing system can also burn the grinding belt.
Since the contoured work-piece that is to be ground must press the rotary grinding belt into the pressure foot the innermost contours of the profile are very hard for the grinding belt to reach whereas the external rounded portions of the profile contour are ground to an increased degree. This is particularly disadvantageous in the case of profile shapes that are covered with a very thin veneer. The constant deformation of the grinding belt when it is pressed into the profile contour of the pressure foot exposes it to a great deal of punishment and this can lead to separation of the abrasive grain which in turn leads to a reduction in the useful life of the grinding belt.
In contrast to the belt grinding machines that operate with a pressure foot in which a large contact surface on the profile that is to be ground is provided by the relatively long pressure foot, with the advantage that the quality of the grinding work that can be achieved is improved, a belt grinding disk with a contoured guide and pressure roller offers a greatly reduced contact surface which corresponds approximately to the contact surface provided by a grinding disk. The contoured guide rollers are for this reason kept at the greatest possible diameter since, if smaller diameters are used limits are imposed on the three dimensional deformation of the grinding belt that passes over the guide roller. Thus it is common to the use of all belt grinding machines with both contoured guide rollers and with contoured pressure feet that it is impossible to grind smaller radii. Only larger and simple radii of a wood profile can be ground.
In addition, belt grinding machines entail the disadvantage that the grinding belt cannot be guided with sufficient accuracy when used for one sided profile contours.
Grinding wood profiles with rotating grinding disks, as opposed to belt grinding machines, does however entail the advantage that small radii can be ground to a relatively true profile. As opposed to this the useful life of the grinding disk is shorter than the useful life of the belt grinding machines and the grinding quality that can be achieved with grinding disks is poorer than the quality that can be achieved using belt grinding machines that work with contoured pressure feet.